1. Field of the Invention
The present invention relates to data communication and, more particularly, to data communication over a universal serial bus (USB) using a driver circuit, system and method that utilizes a higher speed clock to derive a lower speed data transfer across, for example, the USB at a controlled data transition rate, or slew.
2. Description of the Related Art
The following descriptions and examples are given as background only.
The universal serial bus (USB) is implemented within a standardized protocol as an easy-to-use interface for a personal computer. Since then, USBs have gained widespread acceptance. From the user perspective, the benefits of USB include universal plug-and-play and relative ease of use. When a USB peripheral device is plugged into a USB port on a computer or host, the system will auto-detect and auto-configure the peripheral device. In most cases, there is almost no user intervention required.
The USB host and peripheral devices allow numerous types of data communication to various devices that include peripheral devices such as printers, scanners, keyboards, mouse, joysticks, digital cameras, digital video recorders, data acquisition devices, modems, speakers, telephones, storage devices such as Zip drives or any other peripheral or computing device. Within both the host and the peripheral device, a transceiver is required for receiving and transmitting USB-compatible signals across the USB.
The original USB specification was described as version 1.1 or, more specifically, as the Universal Serial Bus Revision 1.1 Specification. The original specification was modified later to accommodate higher speed signals across the USB, and version 2.0 was derived. Both the USB 1.1 and 2.0 interfaces and specifications are available at the USB website, www.usb.org. USB 1.1 focused on making computing easy and was successful in achieving that goal. However, the bandwidth of USB 1.1 proved insufficient for some applications. For example, USB 1.1 provided transmission at a top speed of 12 Mb/s and at some applications proved overly sluggish. Resulting from this problem, USB 2.0 was developed. USB 2.0 can transfer data at 480 Mb/s. However, USB 2.0 protocol requires backward compatibility to the USB 1.1 devices (host and peripheral devices). Thus, USB 2.0 requires transceivers which can send and receive data at 480 Mb/s and also at the USB 1.1 rate of 12 Mb/s full speed and 1.5 Mb/s low speed.
The USB drivers that form part of the USB transceiver for a low speed transmission typically use large resistors and capacitors to implement slew rate control, alternatively known as “edge-rate” control, of the data sent from the driver onto the USB. For example, the USB 2.0 and 1.1 specifications call for a well-defined time at which the data output from the driver onto the USB transitions from a low to a high voltage value or vice-versa. That time of transition is basically demonstrated as an angle or transition rate from logic values 1 to 0 or vice-versa. In driver nomenclature, the rate at which a signal changes from a low to a high voltage value or vice-versa is oftentimes referred to as the slew rate. It is important that when producing an output from a driver, the slew rate is carefully controlled within a defined window, and that window is typically specified by the USB specification.
Using large resistors and capacitors to control the slew rate involves analog methodologies and consumes considerable amount of silicon surface area in order to accommodate the resistors and capacitors. For example, the resistors are often fairly long polysilicon elements and the capacitors can be parallel-plate capacitors, both of which consume integrated circuit area. Moreover, due to process fluctuations, the capacitor plate size, dielectric constant, and thickness can vary from wafer-to-wafer or wafer lot-to-wafer lot. Still further, capacitors and resistors oftentimes perform differently at different temperatures. Therefore, the analog methodology can lead to highly uncontrollable slew rates at the output of the USB driver. Attempts to more tightly control the driver performance may require significantly long design verification time, capacitor trimming, and other means that would deleteriously increase the design cycle time. As the semiconductor fabrication process changes or as the process technology of one vendor differs from another, it is difficult to port the analog design methodology between technologies or vendors. Therefore, the slew rate can vary dramatically among technologies, vendors, and across voltage and temperature fluctuations.
It would be desirable to introduce a driver circuit that is not susceptible to technology, vendor fluctuations, or fluctuations in operating voltage and/or temperature. The desired driver circuit and driving methodology should avoid the conventional analog techniques and the lack of slew rate control offered by such techniques.